1. Field of the Invention
This invention relates to a power supply apparatus capable of efficiently utilizing a built-in battery, and an electronic apparatus using the power supply apparatus.
2. Description of the Related Art
Portable electronic apparatuses such as notebook type personal computers, portable telephone sets, and so forth, have been widely used in recent years. A battery is built in as the power supply of such electronic apparatuses. However, because the voltage of the battery generally drops with the progress of discharge, the battery output has been stabilized by a DC-DC conversion circuit so as to keep the voltage used in the electronic apparatus main body constant.
On the other hand, the electronic apparatuses of the type described above can utilize an external D.C. power supply by utilizing an A.C. adaptor, and a voltage higher than the rated voltage used in the electronic apparatus main body is input in order to charge a rechargeable battery by an external D.C. power supply. For this purpose a voltage reduction type DC-DC conversion circuit is used, which executes a voltage reduction operation for using the battery power supply. When the battery voltage drops, however, the voltage reduction type DC-DC conversion circuit does not operate, and this is the problem to be yet solved.
When the electronic apparatus is operated by the built-in battery without being connected to the external power supply, the battery voltage drops with the progress of discharge, but the input from the battery is regulated by the voltage reduction type DC-DC conversion circuit and outputs a predetermined voltage as a driving voltage of each portion of the electronic apparatus main body such as 5.0 V, for example. On the other hand, in the voltage reduction type DC-DC conversion circuit, the input voltage must be higher than the output voltage, because there is an input/output voltage ratio which is proportional to a voltage drop across transistors and a choke coil in the conversion circuit and to an input/voltage ratio proportional to an ON/OFF ratio of the transistor. Generally, a minimum input voltage required for obtaining a 5.0 V output is about 6.0 V.
On the other hand, when two lithium ion secondary batteries (Li+) of 4.2 V per cell, which will be a predominant battery in the future as the power supply of the electronic apparatuses, are connected in series and are used, the voltage is 8.4 V at the time of the full charge state, but as discharge proceeds, the battery voltage drops and discharge finishes finally at 5.0 V. However, because the voltage reduction type DC-DC conversion circuit does not operate at a voltage below 6.0 V, discharge of the battery must be stopped at 6.0 V.
In this case, though the battery has the capacity to discharge down to 5.0 V, the remaining 1.0 V cannot be used. There is a difference of about 10% between the case where this battery is discharged to 5.0 V and the case where discharge is stopped at 6.0 V. Therefore, if the Li+ battery is used for the voltage reduction type DC-DC conversion circuit requiring the 5.0 V output, only about 90% of the battery capacity can be used and the remaining about 10% becomes useless. In other words, there remains the problem that the battery cannot be used at maximum efficiency.
To cope with this problem, a method has been proposed which connects three lithium ion secondary batteries (Li+) in series, but in this case, the voltage at the time of full charge becomes 16.8 V, and this voltage is close to the limit of withstand voltages of ordinary components of the power supply. This voltage does not cause any problem, in particular, when the battery is discharged, but in consideration of the charging operation of the battery, the voltage necessary for charging the 16.8 V battery is at least 18.0 V and this exceeds the limit of the ordinary components. Thus, another problem develops. Efficiency of the voltage reduction type DC-DC conversion circuit becomes higher when the difference between the input voltage and the output voltage is smaller, and is likely to drop with the increasing difference between the input and output voltages. Therefore, in consideration of voltage reduction type DC-DC conversion efficiency, it is not preferred to set the battery voltage to a high level.
To solve the problem described above, there is a system which uses a booster/reduction type DC-DC conversion circuit and can always output a required voltage both when the battery voltage is higher than the output voltage and when it is lower than the output voltage.
However, whereas power conversion efficiency is from 90 to 95% in the voltage reduction type DC-DC conversion circuit, it is as low as from 75 to 85% in the booster type DC-DC conversion circuit and is extremely low, i.e. about 60%, in the booster/reduction type DC-DC conversion circuit. As described above, when the booster/reduction type DC-DC conversion circuit is used, efficiency of the use of the battery becomes deteriorated. More concretely, we consider the case of the lithium ion secondary battery (Li+), for example. In the case of the voltage reduction type circuit, the loss due to DC-DC conversion is 10% and the unused voltage of the battery at the time of the voltage drop is 10%, or in other words, the total loss is about 20%. In the case of the booster/reduction type circuit, the loss due to DC-DC conversion is 40% and this value is twice as bad as the voltage reduction type with respect to efficiency of the battery use.
To solve the problems of the voltage reduction type DC-DC conversion circuit and the booster/reduction type conversion circuit, a system has been proposed which connects in parallel the voltage reduction type DC-DC conversion circuit and the booster type DC-DC conversion circuit, operates the voltage reduction type DC-DC conversion circuit when the battery voltage is high and operates the booster type DC-DC conversion circuit when the battery voltage is lower than a predetermined voltage.
According to this system, the booster type DC-DC conversion circuit is connected in parallel with the voltage reduction type DC-DC conversion circuit. In the booster type DC-DC conversion circuit, however, the input voltage passes to the output side when the input voltage is higher than the output voltage. Therefore, the input of the booster type DC-DC conversion circuit must be cut off by a switch in such a case. Further, when the conversion circuit is changed over from the voltage reduction type to the booster type, a penetration voltage appears on the output side. In other words, the high input voltage appears as the output voltage.
In the voltage reduction type DC-DC conversion circuit, power can be supplied from a choke coil to a load in both ON and OFF cycles of a switching transistor, but in the booster type DC-DC conversion circuit, power can be supplied from the choke coil to the load only in the OFF cycle of the main switching transistor. As a result, the booster type DC-DC conversion circuit has a time lag from the start of the DC-DC conversion operation until the output of the rated output voltage. Further, the voltage reduction type DC-DC conversion circuit requires the input voltage be somewhat higher than the output voltage, as explained above. As a result, in the system wherein the voltage reduction type and the booster type DC-DC conversion circuits are connected in parallel, there is a gap between the battery voltage at which the voltage reduction type DC-DC conversion circuit can operate normally and the battery voltage at which the booster type DC-DC conversion circuit normally starts its operation.
In a power supply apparatus which connects the voltage reduction type DC-DC conversion circuit in parallel with the booster type DC-DC conversion circuit, there is another system which delays a switching time from the voltage reduction type DC-DC conversion circuit to the booster type DC-DC conversion circuit in order to prevent penetration of the input voltage. In this case, the voltage reduction type DC-DC conversion circuit is operated until the battery voltage drops to a voltage near the output voltage, and thereafter the booster type DC-DC conversion circuit is operated. Accordingly, penetration of the battery voltage becomes less, but because the output voltage drops in the switching period, there remains the problem in stabilization of the output voltage.
In the power supply apparatus wherein the voltage reduction type DC-DC conversion circuit and the booster type DC-DC conversion circuit are connected in parallel, the output voltage supplied to the load becomes discontinuous at the time of switching from the voltage reduction type to the booster type, and a step-like change occurs. Such a step-like change of the voltage results in a fatal problem in the electronic apparatus including a digital data processing circuit comprising semiconductor devices, and invites erroneous operation of the electronic apparatus.